The overall objectives of this research are the theoretical and clinical evaluation of the electrical impedance technique as a harmless, noninvasive method of detecting, measuring, and monitoring physiological events in man and animals. The immediate work proposed concerns elucidating the source or sources of the impedance changes accompanying cardiac activity and critically evaluating the model (a homogeneous "blood cylinder") currently used for calculating stroke volume (and cardiac output) from the impedance record. The effects of intrathoracic fluids such as pulmonary edema, pleural effusion, and pericardial effusion on the calculated values of stroke volume will be determined. Cardiac impedance changes in the presence of known cardiac lesions will be examined to determine if particular impedance waveforms are associated with specific lesions and whether, in the presence of lesions, the impedance technique provides a means of measuring stroke volume. The use of the impedance method as a means of assessing cardiac contractility also will be evaluated.